Various types of ink for inkjet recording have been developed. These include ink prepared by dissolving dyes in an aqueous or non-aqueous medium, dispersing pigments in an aqueous or non-aqueous medium, and solid ink that is thermally dissolved. Ink prepared by dissolving dyes in an aqueous or non-aqueous medium is used because of its color forming ability and stability.
Inkjet printers are used in homes and offices. The quality of images formed by inkjet printing is generally high. To maintain a good inkjet recording performance over a long period of time, aqueous ink for inkjet recording should have the following characteristics. The viscosity, surface tension, and density of the ink should be at appropriate levels, the ink should not clog nozzle inlets of an inkjet recording apparatus when exposed to heat, the ink should not precipitate, and images formed with the ink should have water and light resistance properties.
One of the properties desired of aqueous ink is performance in high-speed printing. Methods of increasing the speed of printers have been focused on the printing head, rather than the ink. Stable ink ejection performance during a high frequency may be related to micro bubbles that are generated in the vicinity of a heater of a thermal head or at an end of a nozzle and attached to a side of the nozzle.
In addition, ink for inkjet recording should be mobile in ink channels of a printing head. However, recently developed printing heads for inkjet printing are small and include narrow and complex channels. Therefore, bubbles may remain in a curved portion or cracks of the ink channels making it difficult for ink to easily flow through the channels. When bubbles are formed in the pathway of ink, the bubbles may flow together with ink and arrive at the printing head. In this case, ink is not easily or is not at all ejected from nozzles.
In particular, in the case of a thermal inkjet printer, when a temperature is increased, bubbles are easily formed due to gas that has been dissolved in ink. In general, ink used in a thermal inkjet printer is heated to a temperature of 25° C. to 80° C. In this temperature range, solubility of gas dissolved in ink may be decreased. Ink may be supersaturated, forming bubbles. The bubbles may clog channels and may hinder flow.
In general, a process of preparing ink is usually accompanied with dissolving of air or other gas in ink. Specifically, gas is dissolved in ink when ink is stirred, when gas diffuses through a specific component of a printing system, and when ink components react with each other.
Ink for inkjet printing may be degasified in a final ink preparation process and then contained in a vessel that has low air permeability. Alternatively, ink for inkjet printing can be filtered to a high degree to remove bubbles and then be contained in a vessel that has low air permeability. These methods described above may be effective only immediately after the gas removal process because the gas removal effect is gradually degraded over time. In particular, when the container including ink is left to sit in a high-temperature condition, gas may be generated even from the degasified ink and bubbles are formed. The bubbles may cause dots missing in the inkjet printer.
In general, bubbles in ink are formed due to gas dissolved in the ink. The amount of the gas can be reduced by removing oxygen (gas) dissolved in ink. Dissolved oxygen can be removed using a physical method or a chemical method. Examples of the physical method include a heating method or a compressing method that is performed at a reduced pressure. However, these physical methods may not be sustainable.
As an example of the chemical method, an oxygen absorber that does not affect quality of ink can be used to effectively remove dissolved oxygen. Examples of an oxygen absorber that chemically reacts with oxygen dissolved ink include Fe; sulfites, such as sodium sulfite, ammonium sulfite, or potassium sulfite; phenols such as pyrogallol; and a reducing agent, such as sodium trithionite hydrazine. To be effectively used in ink, the materials described above should be easily dissolved in ink, should not change color of ink, should not react with oxygen not to generate nitrogen gas, and should not precipitate.
JP No. 52-74406 may disclose that a decrease in a gas concentration in ink can be achieved by adding sulfite to ink so as to decrease the amount of oxygen dissolved in ink. However, sulfite is hardly dissolved in a polyhydric alcohol, which usually acts as a wetting agent in ink, and therefore, may clog an inlet of nozzles.
In addition, a hetero ring compound having an N-substituted amino group and a hydroxyl amine compound have reducing characteristics and can remove oxygen dissolved in ink. However, when these compounds are separately used, the oxygen absorption rate may be low and the amount of oxygen absorbed per unit weight may be low. Therefore, a large amount of the hetero ring compound should be used.